1. Field of the invention
A coating composition which contains a polymeric complex between polyaniline and a polymeric ion. In addition to the said polymeric complex, the compositon contains a water-dispersable binder. The composition is useful as a water-borne paint to be applied onto a metal substrate electrophoretically or non-electrophoretically.
2. Description of Relevant Art
Conductive polymers (xcfx80-conjugated polymers) are potentially useful as a polymeric coating materials to impart special electrical, optical and electroactive properties to coated surfaces. When used as a coating on metals it can impart protection against corrosion of the metals (Wessling DE4334628, Kinlen U.S. Pat. No. 5,532,025). The electrically conductive form of the conducting polymers can also be coated on non-conductive surfaces to render the surfaces electrically conductive. Examples of the xcfx80-conjugated polymers are polyaniline, polypyrrole, polyacetylene, polythiophene etc.
The xcfx80-conjugated polymers are electrically conductive when they are doped by ionic compounds. In the electrically conductive state, the xcfx80-conjugated polymer backbone is a polycation. The positive charge on the xcfx80-conjugated polymer backbone is the mobile charge that leads to electrical conductivity. The dopants are the counter ions that balance the positive charges. The difficulties in using conventional conducting polymers for coatings are associated with two of their properties; (1) they are unstable in their doped state and (2) they lack processability. The reason for the lack of processability comes from the fact that the conducting polymers are xcfx80-conjugated polymers. The delocalized xcfx80 electronic structure leads to a stiff polymer chain and strong inter-chain attraction. Thus, the conventional conducting polymers cannot be easily dissolved, melted or blended with other polymers.
The lack of material stability comes from the fact that the ionic dopants are easily lost or segregated from the xcfx80-conjugated polymers. Examples of the dopants used in the prior art include hydrogen chloride, p-toluene sulfonic acid, 4-dodecylbenzne sulfonic acid, and dinonylnaphthaienedisulphonic acid (Jen et al., U.S. Pat. No. 5,069,820, Dec. 3, 1991; Elsenbaumer, U.S. Pat. No. 5,160,457, Nov. 3, 1992; Cao et al., U.S. Pat. No. 5,232,631, 1993; Kinlen U.S. Pat. No. 5,567,356, Oct. 22, 1996). When these conducting polymers are exposed to heat, water, solvents and/or moisture, these molecular dopants are lost. Once the dopants are lost, the polymer loses its electrical conductivity and its electroactivity. The loss of dopants occurs either during the manufacturing process or during the service life of the coated product. In certain cases, molecular anions with bulky organic groups are used to reduce the rate of loss of the dopant. This only slows down the rate of dopant is loss, it does not eliminate the problem. Even when the dopes are not lost, the electrical conductivity can be lost due to the diffusion of dopants at a microscopic length scale. The detachment of the dopants from the xcfx80-conjugated polymer backbone at a microscopic length scale (0.1 xcexcm length) leads to dedoping. A microscopic scale phase segregation between the polymer and the dopant is easily promoted by heat or solvent. The molecular dopants tend to segregate from the vicinity of the polymeric chain of the xcfx80-conjugated polymer backbone which results in a loss of the desirable properties.
A problem with the conventional xcfx80-conjugated polymers is that they are brittle, hard and solid. In coating applications, the conventional xcfx80-conjugated polymers do not adhere to the surface of the substrate. Thus the xcfx80-conjugated polymers are blended with an insulating, non-conductive resin to form a mixture that could be adherent to the surface of a substrate. See U.S. Pat. Nos. 5,532,025, 5,543,084 and 5,556,518. When the conducting polymer is imbedded in the matrix of a non-conducting polymer such as an epoxy, polyurethane, polyacrylate or alkyd binders, the rate of dopant loss is reduced in the macroscopic level (e.g 0.1 mm length), but the problem of segregation at a microscopic length scale (e.g. 0.1 xcexcm length) is not eliminated. The electroactive properties will show signs of degradation over a period of several months. For a number of applications, the material stability is not good enough. In addition to the problem with the service life of coatings or blends of these xcfx80-conjugated polymers, there are problems with the manufacturing process.
The dopants are easily lost during the manufacturing process either because of heat or because of contact with water or polar solvents. For example, U.S. Pat. No. 5,543,084 discloses a method for electrocoating a blend of epoxy and polyaniline. The conductive polymer PANI-PTSA (polyaniline doped by p-toluenesulfonic acid) was mechanically blended in aqueous solution and then electrophoretically coated on metal. From the disclosure it is evident that the anionic dopant of PANI-PTSA was lost before the xcfx80-conjugated polymer was co-deposited with epoxy. A redoping by immersing the coating in camphor sulfonic acid was needed to restore polyaniline to its electrically conductive state. It is expected that the dopants incorporated by redoping will be easily dedoped again by either heat or by exposure to moisture.
Coatings that use undoped polyaniline (emeraldine base) have been disclosed in the literature (McAndrew et al. U.S. Pat. No. 5,441,772, and Epstein et al. U.S. Pat. No. 5,824,371). These xcfx80-conjugated polymers without dopant are nonconductive because there is no charge carrier on the polymer backbone. For most applications it is essential to maintain the xcfx80-conjugated polymers in the electrically conductive state. Thus it is desirable to have an electrically conductive polymer that is both processable and is stable against the loss of dopants.
An alternative to the above mentioned remedies is to synthesize a molecular complex of the xcfx80-conjugated polymer and a polymeric dopant. If the polymeric dopant is strongly bonded to the xcfx80-conjugated polymer the dopant will not be easily lost during the manufacturing process and the service life of the conducting polymer. A method was previously disclosed for synthesizing processable conducting polymers with stable dopants (Liu et al. U.S. Pat. No. 5,489,400). In this disclosure, a template-guided chemical polymerization was used to obtain a polymeric complex that contained a stand of polyaniline and a strand of a polyelectrolyte. The reaction product is a non-covalently bonded molecular complex between a conducting polymer and a polyelectrolyte. The molecular complex contains the two linear chains of the component polymers bonded in a side-by-side fashion. The complex is a double-strand synthetic polymer. When polyaniline is the conductive strand, dsPAN designates the double-strand polyaniline. Compared with the double-strand biopolymer, DNA, the synthetic dsPAN is less ordered in structure and is generally not in a helical conformation. Examples of the polyelectrolytes are poly(styrenesulfonic acid) and poly(acrylic acid). Since the two strands of polymers are bonded strongly, these polymeric complexes are stable and do not dedope easily.
The dsPAN disclosed in this ""400 patent is one of three types. The first type is a water-soluble polymeric complex of polyaniline. This type of dsPAN is not suitable for anticorrosion coating applications because a pure dsPAN coating is redissolved in contact with water therefore the coating is lost in rain or humid air. It is conceivable that the water-soluble dsPAN can be incorporated in a polymeric binder that prevents water dissolution of the coating. The hyrophilicity of this type of dsPAN is, however, still a problem for corrosion protection. The coating will absorb moisture or swell in water and thus reduce the adhesion of binder to the metal substrate.
A second type of dsPAN disclosed was an insoluble solid that precipitates from the aqueous reaction medium. This type of dsPAN can only be mixed with the binder by vigorous mechanical mixing (in a manner similar to that used for blending single-strand PANI-PTSA with epoxy described in Example 13 of Kinlen et al. U.S. Pat. No. 5,543,084). Although a blend made in this manner overcomes the problem of dedoping in U.S. Pat. No. 5,543,084, it is still not ideal. The dispersion contains large and brittle particles. The particles are not small enough for optimal polymer-metal interaction even when the precipitated particles are ground with a ball mill. The large particles do not xe2x80x98wetxe2x80x99 the metal surface. Another problem is that the mechanically stirred suspension is not a stable dispersion. It is difficult to maintain a uniform and stable suspension for large scale industrial production.
The third type of dsPAN disclosed in the ""400 patent is a colloidal suspension of small particles. Although the particle size is suitable for the electroactive polymer to interact with the metal surface to impart protection of the metal, the concentration of the colloidal particles in water is quite low (less than 1 gm of colloidal particles per liter of water). This low concentration is incompatible to the preferred high-solid content coating formulation.
PCT Publication WO 97/03127 discloses a chemically modified dsPAN that is soluble in polar organic solvents and can be applied to metal surfaces as a paint. The coating disclosed protected metals from corrosion. These organic soluble dsPANs overcame the water absorption (swells problem of the water-soluble dsPAN disclosed in Liu U.S. Pat. No. 5,489,400. This type of dsPAN is suitable for use as either a solvent-based paint or as a blend with hydrophobic epoxy oligomer for a thermoset coating. This type of dsPAN is not disperable in water to make a stable mixture with water-borne epoxy. Thus this type of dsPAN is not suitable for a water-borne coating application.
Electrophoretic deposition of resins on metals provides excellent corrosion protection for steel used in automobiles and appliances. Using a conducting polymer as additive in the electrophoretic coating bath should enhance the effectiveness for protecting the metals from corrosion. Electrophoretic coatings containing single-strand conducting polymers are disclosed in WO93/14166 and U.S. Pat. Nos. 5,128,396, 5,543,084, 5,556,518. However, the dopants used in these disclosures are non-polymeric small molecular ions which have the disadvantage of dedoping in the e-coat processing and dedoping of the coated metal due to heat or moisture.
The present invention comprises a composition that advantageously avoids the problem of dedoping common to all the single-strand conducting polymers. A polymeric complex of polyaniline is dispersed in water as latex-like small particles. The polymeric complex is hydrophobic enough so that the problem of water-absorption and swelling of the painted surface is avoided. The particle sizes of the latex-like suspension are small enough so that the xcfx80-conjugated polymers may interact with the metal surface effectively. The percentage of the xcfx80-conjugated polymers in the non-conducive binder can be low enough so that the mechanical strength of the coating is essentially the same as the coating without the xcfx80-conjugated polymer.
The invention comprises a family of two-component polymeric complexes of xcfx80-conjugated polymers that are suitable for water-borne coating applications. The invention also embodies the process of making the polymeric complexes, the use of the polymeric complexes in an anti-corrosion formulations, a family of coating compositions containing the polymeric complexes, the method of applying the coating composition on a surface and the coating compositions per se.
More particularly, the present invention comprises:
1. A water-borne coating composition comprising a polymeric complex between a xcfx80-conjugated polymer, a polymeric ion (that serves as a dopant for the xcfx80-conjugated polymer), and a non-conductive polymer (that serves as a binder or resin for coating).
2. A water-borne coating composition as in (1) where the xcfx80-conjugated polymer and the polymeric dopant are strongly (non-covalently) bonded to form a molecular complex.
3. A water-borne coating composition as in (2), where the polymeric molecular complex has substantial any with the non-conductive polymeric binder so that the water-borne dispersion will not segregate, and the dried coating film is homogeneous in composition.
4. A composition described in (3) that can be electrophoretically deposited on a metal substrate to form a coating that contains a polymeric dopant, and a non-conductive polymeric binder. All three components are electrodeposited onto the metal surface with negligible loss of dopants.
5. A coating composition described in (3) or (4) that has enhanced anticorosion performance compared with a coating composition that does not contain a polymeric complex of the xcfx80-conjugated polymer.
6. A coating composition of (3), (4) or (5), in which the non-conductive polymer is a thermoset polymer and the polymeric complex of the xcfx80-conjugated polymer serves the dual function of an anticorrosion additive and a crosslinking agent.
7. Optionally, the said electroactive (or conducting) polymeric complex is used as an additive to a commercial electrocoat formulation that contains additional cross-linking agents.
8. An electrocoating process that allows deposition of the coating composition onto a metal surface.
9. An electrocoating process that allows cathodic deposition of the coating composition onto a metal surface.
10. An electrocoating process of (9) that forms a protective coating on the surface of aluninum alloys.
11. An electrocoating process of (9) that forms a protective coating on the surface of steel.